Alginate is a naturally occurring polysaccharide composed of D-mannuronic acid (M block) and L-guluronic acid (G block). Alginate is derived from the cell wall of different species of brown algae. A unique property of alginate is that it forms a hydrogel under mild conditions. The major gelation mechanism in alginate is crosslinking of the polysaccharide carboxyl groups by positively charged metal cations [Nussinovitch, A., Hydrocolloid applications: gum technology in the food and other industries. Blackie Academic & Professional London, 1997]. Alginate has been extensively used for encapsulation and immobilization of various types of materials including, inter alia, inorganic ions, organic molecules (immobilization of microorganisms, encapsulation of hydrophilic and hydrophobic components) and living cells. Technological fields widely utilizing alginate are pharmacology, agriculture, food industry and water treatment. After dissolution alginate can be used to thicken (increase viscosity) of fluids (water). For example, propylene glycol alginate (PGA) increases the viscosity of many salad dressings, due to its stability in low pH.
A variety of hydrocolloids have been studied for their potential use as carriers for the controlled release of compounds, including drugs and agro-chemical compounds, such as those used for pest control. Several studies have focused on alginate-based carriers, revealing some difficulties. For one, the loading efficacy of the active ingredient (drug, agro-chemical compound) is too low due to its leakage into the cross-linking solution [El-Kamel A H, Al-Gohary O M N, Hosny E A. 2003, J Microencapsul 20(2):221-225; Liu P, Krishnan T R. 1999, J Pharm Pharmacol 51(2):141-149].
Alginate has also been one of the most commonly used biomaterials for cell encapsulation, probably due to its natural origin and excellent biocompatibility. The gelling property of alginate, has been utilized to prepare cell-loaded alginate beads by electrospray and microfluidic flow focusing. However, microbeads consisting solely of alginate were not sufficiently effective in protecting the encapsulated cells from being attacked by the host immune system. Therefore, polylysine (PLL) coated alginate microbeads and alginate-PLL-alginate or APA microcapsules were developed to allow further permeability control [F. Lim, A. M. Sun, Science, 210, 21, 1980, 908-910; G. Orive, S. K. Tam, J. L. Pedraz and J. P. Halle, Biomaterials, 2006, 27, 3691-3700; S. K. Tam, S. Bilodeau, J. Dusseault, G. Langlois, J. P. Halle and L. H. Yahia, Acta Biomater., 2011, 7, 1683-1692]. Alginate counteracts rudimental charges of chitosan, thereby providing a stable shell. However, APA microcapsules have a drawback of inducing inflammatory cytokine release from host immune cells.
Conventional treatment technologies for the removal of heavy metals from aqueous solutions are expensive and generate huge quantities of toxic chemical sludge [Shawky, H. A., 2011, Journal of Applied Polymer Science 119, 2371-2378]. Many different technologies aimed at mitigating the problem have been proposed, and can be classified into three major categories, including chemical treatments, such as precipitation and ion-exchange filters; biological treatments, specifically biosorption; and physical methods, such as, for example, adsorption and membrane filtration [Hashim, M., Mukhopadhyay, S., Sahu, J. N., Sengupta, B., Journal of environmental management 92, 2355-2388, 2011; Hua, M., Zhang, S., Pan, B., Zhang, W., Lv, L., Zhang, Q., Journal of Hazardous Materials 211, 317-331, 2012]. Another efficient alternative includes biotechnological processes based on polysaccharides' ability to adsorb heavy metals [Papageorgiou, S., Kouvelos, E., Katsaros, F., Desalination 224, 293-306, 2008; Hua et al., 2012].
Alginate may be used to eliminate metal ions from effluent via two possible mechanisms: the alginate can immobilize other types of heavy-metal adsorbents, or it can serve as the chelating agent itself [Plazinski, W., Environmental Science and Pollution Research 19, 3516-3524, 2012]. Under the latter mechanism, alginate is already partially crosslinked, mainly using calcium cations, and the heavy-metal cations are sequestered from the solution, bonding to the alginate's remaining free carboxyl groups or replacing some of the calcium cations [Singh, L., Pavankumar, A. R., Lakshmanan, R., Rajarao, G. K., Ecological Engineering 38, 119-124, 2012]. Lead-adsorption efficiency by calcium-alginate has been reported to be 240 mg Pb2+/g [Stewart, T. J., Yau, J.-H., Allen, M. M., Brabander, D. J., Flynn, N. T., Colloid and Polymer Science 287, 1033-1040, 2009], 144 Pb2+/g [Yakup Arica, M., Arpa, , Ergene, A., Bayramo{hacek over (g)}lu, G., Gen Ö., Carbohydrate Polymers 52, 167-174, 2003] and 130 Pb2+/g [Lagoa, R., Rodrigues, J., Applied biochemistry and biotechnology 143, 115-128, 2007].
Other hydrocolloid materials have also been studied for heavy metal absorption. The Ni2+ accumulation in batch mode from diluted solutions by gel beads of gellan gum (GG), alginate, κ-carrageenan, agar, agarose, and two mixtures of GG+agar was investigated, where gel beads of GG were stable, easily obtainable and showed the highest Ni2+ accumulation [Lázaro N1, Sevilla A L, Morales S, Marqués A M, Water Research 05/2003; 37(9):2118-26].
EP1063201 is directed to a method of removing harmful ions contained in water, such as copper, zinc, cadmium, chromium or lead, characterized in that an anionic group-containing hydrophilic polymeric substance is dispersed in said water in an undissolved state, and said harmful ions are insolubilized while maintaining said anionic group-containing hydrophilic polymeric substance in the undissolved state.
In all of the above technologies, alginate is typically used in a solid or semi-solid form, in which alginate is crosslinked by divalent metal cations, most commonly by calcium ions. To the best of the inventors' knowledge, no non-crosslinked alginate as a core within a liquid-core capsule, wherein the liquid-core capsule is produced in a one-step process, has been reported for use in water purification. Recently, liquid-core hydrocolloid capsules were reported for use in cell encapsulation. ACA capsules with liquid alginate core were produced by a multi-step process including forming conventional ACA solid beads and liquefying the alginate core [Zhang et al., 1002|J. Mater. Chem. B, 2013, 1, 1002-1009].
There remains an unmet need for the cost-effective, environmentally friendly and easily producible hydrocolloid-based materials for use in the removal of toxic heavy metals from aqueous media, drug encapsulation, and food and agricultural technology.